1. Field of the Invention
The present invention relates generally to methods and apparatus for drilling earthen formations. More particularly, the present invention relates to methods and apparatus for drilling earthen formations for the recovery of petroleum using high-pressure, reduced solid content liquid.
2. Background Information
It is a long-standing practice in the rotary drilling of wells to employ a drilling fluid. In most cases, the drilling fluid is a dense, filter-cake-building mud to protect and retain the wall of the borehole. The mud is pumped through the tubular drillstring, exits nozzles in the drill bit, and is returned to the surface in the annulus between the drillstring and the sidewall of the borehole. This fluid cools and lubricates the drill bit as well as providing a hydrostatic fluid column to prevent gas kicks or blowouts, and builds filter cake on formation in the sidewall of the borehole. The drilling fluid exits the bit through nozzles to strike the bottom of the well with a velocity sufficient to rapidly wash away the cuttings created by the teeth of the bit. It is known that the higher velocity of the fluid, the faster will be the rate of drilling, especially in the softer formations that can be removed with a high-velocity fluid.
Although mud hydraulics using higher nozzle velocities are well-known to beneficially affect the rate of penetration of the bit, generally the drilling fluid is not employed as a primary mechanism for the disintegration of formation material. One reason for this is that conventional drilling muds are quite abrasive, even though there is effort to reduce the amount of abrasives. The pressures required to generate hydraulic horsepower sufficient to actively disintegrate formation material cause extreme abrasive wear on the drill bit, especially the nozzles, and associated drillstring components when abrasive particles are in the drilling fluid. Use of clear water or a non-abrasive fluid would solve the abrasion problem, but the density and characteristics of such fluids cannot substitute for the dense, filter-cake-building drilling mud in formations that are porous or tend to slough-off. Nor can clear water be used when high-pressure gas may be encountered and a high-density fluid is required to prevent a blowout.
Attempts have been made to employ a high-pressure, reduced solid content drilling fluid together with a dense, filter-cake-building drilling mud to achieve the advantages of both. U.S. Pat. No. 2,951,680, Sep. 6, 1960, to Camp discloses a two-fluid drilling system in which an inflatable packer is rotatably coupled to the drillstring just above the drill bit. In drilling operation, the packer is inflated and the annulus between the drillstring and the borehole wall above the packer is filled with conventional drilling mud. Gaseous or reduced density drilling fluid is pumped down through the drillstring and exits a nozzle in the bit. The packer prevents mixing of the drilling and annulus fluids. The cutting-laden drilling fluid is returned to the surface through a port in the sidewall of the drillstring below the packer and a conduit formed within the drillstring. The presence of a packer near the drill bit in the drillstring poses design and reliability problems. Additionally, the cutting-laden drilling fluid is returned through a tortuous passage in the drillstring, which is likely to become clogged with cuttings.
U.S. Pat. No. 3,268,017, Aug. 23, 1966, to Yarbrough discloses a method and apparatus for drilling with two fluids in which a two-tube, concentric drillstring is employed. Clear water is employed as the drilling fluid and is pumped down through the inner tube of the drillstring and exits the bit. A wall-coating drilling mud or fluid is maintained in the annulus between the drillstring and the borehole. Cutting-laden drilling fluid is returned to the surface through the annulus defined between the inner and outer concentric tubes of the drillstring. The height of the column of wall-coating drilling mud is monitored and pressure in the drilling fluid is increased responsive to pressure increases resulting from changes in the hydrostatic pressure associated with the column of wall-coating liquid between the drillstring and borehole wall. Returning the cutting-laden fluid in an annulus between inner and outer conduit in a drillstring would be problematic because the annulus would tend to clog and would be very difficult to clean. Additionally, monitoring the pressure exerted by the annulus fluid by measuring its height in the wellbore would be extremely difficult to accomplish if annulus fluid or drilling mud is continuously pumped into the annulus, which is necessary to maintain the annulus fluid or drilling mud over the entire length of borehole as drilling progresses.
U.S. Pat. No. 4,718,503, Jan. 12, 1988, to Stewart discloses a method of drilling a borehole in which a drill bit is coupled to the lower end of a pair of concentric drill pipes. A first low-viscosity fluid, such as oil and water, is pumped down through the inner drill pipe and returned to the surface through the annulus between the inner and outer drill pipes. A column of annulus fluid or drilling mud is maintained stationary in the annulus formed between the borehole wall and the outer of the drill pipes. When it becomes necessary to make-up a new section of drill pipe, filter-cake-building drilling mud is pumped down the inner drill pipe to displace the clear drilling fluid, wherein only the dense, filter-cake-building annulus fluid or drilling mud occupies the borehole. Such a procedure for the make-up of new sections of drill pipe is extremely unwieldy, and in practice is uneconomical.
A need exists, therefore, for a method and apparatus for drilling with a reduced density drilling fluid while maintaining a dense, filter-cake-building annulus fluid in the annulus that is commercially practical.